Anatomical and histochemical studies of foetid-odor emission osmophores on the labella of two Orchidantha (Lowiaceae) species

Orchidantha (Lowiaceae) flowers exhibit a fascinating pollination strategy, employing foul scents to attract pollinators specifically to the prominent median petal, known as the labellum. In Thailand, two distinct Orchidantha species, O. foetida and O. siamensis, exhibit divergent floral morphologies. This variation includes differences in the structure of the labellum, the overall form and coloration of the flowers, and their geographic distribution. This study investigates the secrets hidden within their foul-odored labella, revealing the intricate structures and chemical compositions of their scent-producing glands, known as osmophores. Employing a combination of microscopic techniques, we uncovered crucial differences in the labella anatomy of these two species. Only adaxial labellum of O. foetida functions as osmophores with papillate and non-papillate cells, while labellum of O. siamensis exhibits osmophoric activity on both the adaxial and abaxial surfaces with a similar cellular pattern of papillate and non-papillate cells. Interestingly, O. foetida alone harbors osseous aerenchyma within its labella, potentially aiding in the metabolic processes fueling scent production. Histochemical analysis identified scent-related compounds accumulating within the osmophoric cells of both species including alkaloids, carbohydrates, lipids, mucilage, phenolic, and terpenes. These findings shed light on the diverse traits of labellum of Orchidantha species employ in their pollination strategy. The subtle variations in osmophore morphology and chemistry between O. foetida and O. siamensis may well reflect adaptations to distinct pollinator preferences.